This invention relates to an image-receiving element for use in photographic film units of the diffusion transfer type. More particularly, it relates to an image-receiving element especially adapted to use in so-called "peel-apart" diffusion transfer film units which include an image-receiving element designed to be separated after photographic processing.
Photographic film units of the diffusion transfer type, including the aforementioned diffusion transfer "peel-apart" film units, are well known and have been described in numerous patents, including, for example, U.S. Pat. Nos. 2,983,606; 3,345,163; 3,362,819; 3,594,164; and 3,594,165. In general, diffusion transfer photographic products and processes involve film units having a photosensitive system including at least one silver halide layer, usually integrated with an image-providing material, e.g., an image dye-providing material. After photoexposure, the photosensitive system is developed, generally by uniformly distributing an aqueous alkaline processing composition over the photoexposed element, to establish an imagewise distribution of a diffusible image-providing material. The image-providing material is selectively transferred, at least in part, by diffusion to an image-receiving layer or element positioned in a superposed relationship with the developed photosensitive element and capable of mordanting or otherwise fixing the image-providing material. The image-receiving layer retains the transferred image for viewing. In diffusion transfer photographic products of the so-called "peel-apart" type, the image is viewed in the image-receiving layer upon separation of the image-receiving element from the photosensitive element after a suitable imbibition. In other products, such separation is not required.
Image-receiving elements especially adapted for use in "peel-apart" diffusion transfer film units have typically embodied a combination of particular layers on a suitable substrate material, each of the layers providing specific and desired functions adapted to the provision of the desired photographic image by diffusion transfer processing. Thus, a preferred image-receiving element has typically comprised a support material (preferably, an opaque support material carrying a light-reflecting layer for the viewing of the desired transfer image thereagainst by reflection); a polymeric acid-reacting (neutralizing) layer adapted to lower the environmental pH of the film unit subsequent to substantial transfer image formation; a spacer or timing layer adapted to slow the diffusion of the alkali of an aqueous alkaline processing composition toward the polymeric neutralizing layer; and an image-receiving layer to receive the transferred photographic image. Such preferred structure is described, for example, in the aforementioned U.S. Pat. No. 3,362,819 and is illustrated in other patents, including U.S. Pat. Nos. 4,322,489 and 4,547,451.
Various materials have been described as being suited to application as a spacer or timing layer positioned between the polymeric acid-reacting layer and the image-receiving layer of an image-receiving element of the aforedescribed type. Thus, in the aforementioned U.S. Pat. No. 4,322,489, reference is made to the use of polyvinyl alcohol, gelatin or other polymers through which alkali may diffuse to the polymeric acid-reacting layer. The presence of such a timing layer between the image-receiving layer and the acid-reacting layer effectively controls the initiation and the rate of capture of alkali by the acid-reacting layer. Other materials suitable for the formation of timing layers and the advantages thereof in diffusion transfer systems are described with particularity in U.S. Pat. Nos. 3,362,819; 3,419,389; 3,421,893; 3,455,686; 3,577,237; and 3,575,701.
It has been disclosed that advantages in diffusion transfer processing can be realized by employing as a timing layer a polymeric material which functions as an alkali-impermeable barrier for a predetermined time interval and which then converts to a relatively alkali-permeable condition upon occurrence of a predetermined chemical reaction in the timing layer to allow access of the alkali to the neutralization layer in a rapid and quantitatively substantial fashion. The capacity of the timing layer to prevent passage or diffusion of alkali therethrough for a predetermined length of time during the processing of the film unit, and the capacity of the layer to convert over a short time period to a condition of substantial permeability to alkali, allows the layer to serve as an effective diffusion control layer. The timing layer thus acts as a "hold-release" layer, in that, alkali subject to diffusion control by the timing layer is "held" in place for a predetermined period of time and then "released" in substantial quantity over a relatively short time period, i.e., allowed to rapidly diffuse through the layer. This desirable "hold-release" behavior may be contrasted with the behavior of timing layers which do not undergo a precipitous change in permeability but, rather, are initially permeable to alkali to some degree and which, thus, allow a slow leakage of alkali from the start of processing, gradually becoming more permeable during the processing interval.
The chemical reaction mechanism utilized in the production of a timing layer exhibiting desired "hold-release" behavior can be a beta-elimination reaction which is activated by the alkali of the alkaline processing composition. Examples of polymeric materials which undergo an alkali-initiated beta-elimination reaction, and which can be used as timing layers of the "hold-release" type are known and are described in U.S. Pat. Nos. 4,201,587; 4,297,431; 4,391,895; 4,426,481; 4,458,001 and 4,461,824. Timing layers which are converted from a condition of impermeability to alkali to a condition of substantial permeability thereto as a function of a predetermined hydrolysis reaction, are also useful and are described in U.S. Pat. No. 4,547,451.
The use of timing layers of the aforedescribed "hold-release" type provides advantages in color saturation, notably by preventing premature reduction of environmental pH in the film unit during processing and by allowing substantial dye-image transfer to occur at elevated pH before a substantial and predetermined pH reduction. These benefits are, in general, obtained by employing a timing layer of the aforedescribed character which typically will be a relatively water-impermeable layer which is non-sorptive of water and which is coated as a thin layer of a thickness adapted to the particular timing requirement of a photographic system. Such a layer will, in general, be provided conveniently by coating a latex of polymeric material having the predetermied diffusion control properties. While substantial benefits are realized by utilizing timing layers of the aforedescribed type, deficiencies have, nonetheless been observed.
For example, there has been observed a tendency for the image-bearing layer to be incompletely adhered to the timing layer, such that, the application of slight pressure to the photograph freshly separated from the photosensitive element, causes a shifting or smearing of the layer, thus, producing image distortion. In addition, salt materials have been detected in the image-bearing layer. These salt materials contribute to haze and prevent the realization of desirable maximum dye densities.